code: def testStdDev(self):
      values = [1, 1, 2, 3, 5]
      seqDS = dataseries.SequenceDataSeries()
      stdDev = stats.StdDev(seqDS, 2)
      for value in values:
          seqDS.append(value)
  
      self.assertEqual(stdDev[0], None)
      self.assertEqual(stdDev[1], numpy.array([1, 1]).std())
      self.assertEqual(stdDev[2], numpy.array([1, 2]).std())
      self.assertEqual(stdDev[3], numpy.array([2, 3]).std())
      self.assertEqual(stdDev[4], numpy.array([3, 5]).std())
  
file_path: pyalgotrade/testcases/technical_stats_test.py
imports: import numpy
  from .None import common
  from pyalgotrade.technical import stats
  from pyalgotrade import dataseries
label_desc: Use image node_id 2 for calling the TestCase obj's underlying member method code with example usage: obj.testStdDev() without return types